1. Field of the Invention
The present invention relates to a regeneration method for a sugar solution purification system for deionizing and decolorizing a sugar solution such as a starch sugar solution, and in particular to a regeneration method for ion exchange resins in a mixed-bed type sugar solution deionization system which uses a strongly acidic cation exchange resin and a basic anion exchange resin.
2. Description of the Related Art
When a starch is hydrolyzed by an acid or by an enzyme, various types of starch sugars (which refers to the sugars manufactured from starch as the raw material) are obtained depending on various hydrolysis conditions. The hydrolysis of starch can be divided into two steps, liquefaction and saccharification. A starch sugar solution can be obtained by starch saccharification, but the starch sugar solution contains some impurities. In order to remove these impurities, a starch sugar solution refining step is taken after the starch saccharification step.
When the starch sugar solution is refined, ion exchange processes are performed as a post-process following such refining steps as carbonation, granular active carbon filtration, and bone char filtration. The ion exchange processes include an ion exchange process for decolorizing and an ion exchange process for deionization.
The ion exchange process for deionization generally consists of two-bed type front deionization system which uses a strongly acidic cation exchange resin layer and a weakly basic anion exchange resin layer, and a mixed-bed type polishing deionization system which uses a strongly acidic cation exchange resin and a Type II strongly basic anion exchange resin. Most of the impurities such as the salts and color bodies are removed from the original solution at the front deionization system and the polishing deionization system effects final deionization, decolorization, and pH adjustment. The deionization process is widely used throughout the industry and is superior as a refining method for the sugar solution in that a high purity sugar solution can be obtained. This is attributed to superb performance of the polishing mixed-bed ion exchange system. In recent years, methods have been proposed in which a weakly basic anion exchange resin or a Type I strongly basic anion exchange resin is used in place of the Type II strongly basic anion exchange resin in the polishing mixed-bed ion exchange system.
Regeneration of ion exchange resins in the mixed-bed type sugar solution polishing deionization system using a strongly acidic cation exchange resin and a basic anion exchange resin is currently performed as follows. First, after the sugar solution has been passed through the mixed-bed ion exchange column to a certain endpoint, the strongly acidic cation exchange resin is separated to the lower layer and the basic anion exchange resin is separated to the upper layer within the mixed bed column. This separation of the two resins which are in mixed condition is achieved by backwash by dint of the relative density difference between the resins. An acid regenerating agent such as a hydrochloric acid solution is passed upward through the strongly acidic cation exchange resin at the lower layer to regenerate the strongly acidic cation exchange resin. Similarly, an alkali regenerating agent such as a sodium hydroxide solution is passed downward through the basic anion exchange resin at the upper layer to regenerate the basic anion exchange resin. While the regenerant is being passed through one of the two resin layers, countercurrent water is passed through the other resin layer, and both waste solution from resin regeneration and water are discharged from a collector provided at the separation boundary of the resins. There also exists a method for simultaneously passing the two regenerants through respective resin layers. After the completion of their regeneration, the resins are mixed to again form a mixed bed.
In the mixed-bed type sugar solution deionization system using a strongly acidic cation exchange resin and a basic anion exchange resin, with the conventional regenerating method of passing an acid regenerating agent through the strongly acidic cation exchange resin and an alkali regenerating agent through the basic anion exchange resin, there were some cases where, after a few cycles of regeneration step, the conductivity of the sugar solution treated by the mixed-bed increases. The inventors analyzed the salts contained within the treated sugar solution having a higher conductivity and concluded that there was a leakage of amphoteric organic materials such as amino acids from the mixed-bed system to the treated sugar solution.
One object of the present invention is to provide a regeneration method for a mixed-bed type sugar solution deionization system which can effectively prevent the increased conductivity of the treated sugar solution.
The inventors examined the cause of the leakage of amphoteric organic materials such as amino acids into the treated sugar solution, and concluded that, at the initial stage, the amphoteric organic materials such as amino acids in the sugar solution are adsorbed by both the strongly acidic cation exchange resin and the basic anion exchange resin, but because the ion adsorption capability of the basic anion exchange resin is weaker than that of the strongly acidic cation exchange resin and is weakened as the ion exchange deionization is repeated, the amphoteric organic materials such as amino acids in the sugar solution are eventually adsorbed by the strongly acidic cation exchange resin alone.
As described above, the amphoteric organic materials such as amino acids in the sugar solution are mostly adsorbed by the strongly acidic cation exchange resin. However, in the conventional regenerating method using an acid regenerating agent, the amphoteric organic materials tend not to desorbed from the strongly acidic cation exchange resin. The inventors suspect that this is due to the cation portion of the amphoteric organic materials such as the amino acids being adsorbed by the strongly acidic cation exchange resin when the amphoteric organic materials are adsorbed by the strongly acidic cation exchange resin. Because of this, passing of an acid regenerating agent such as hydrochloric acid will not cause the amphoteric organic materials themselves to be desorbed, and only the anion portion such as the carboxylic acid radical which was not captured is ion exchanged.
The inventors have thus examined various means for desorbing the amphoteric organic materials such as the amino acids from the strongly acidic cation exchange resin, and concluded that by passing an alkali regenerating agent such as sodium hydroxide through the strongly acidic cation exchange resin, the amphoteric organic materials such as the amino acids can efficiently be desorbed from the strongly acidic cation exchange resin. This is suspected to be due to desorption of the cation portion of the captured amphoteric organic materials such as the amino acids from the strong acidic cation exchange resin when the alkali regenerating agent contacts the strongly acidic cation exchange resin, causing the entire amphoteric organic materials to be desorbed.
The present invention is based on this discovery, and provides a regenerating method of a mixed-bed type sugar solution deionization system for regenerating ion exchange resins in the mixed-bed type sugar solution deionization system filled with a strongly acidic cation exchange resin and a basic anion exchange resin, comprising the steps of contacting an alkali regenerating agent with both the strongly acidic cation exchange resin and the basic anion exchange resin, and then contacting an acid regenerating agent with the strongly acidic cation exchange resin.
In the regenerating method of the present invention, by passing an alkali regenerating agent through both the strongly acidic cation exchange resin and the basic anion exchange resin, the basic anion exchange resin is regenerated while amphoteric organic materials such as the amino acids captured on the strongly acidic cation exchange resin are desorbed, and then, by passing an acid regenerating agent through the strongly acidic cation exchange resin, the cation exchange resin is regenerated, resulting in prevention of the leakage of amphoteric materials such as amino acids into the treated sugar solution and stabilization of the quality of the treated sugar solution.